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Blowing out from .243 using shotgun powder will result in the longest and most uniform necks, as well as the thinner than from .308 brass. If you have a custom chamber this could work.... it's also a pita.

Necking up using a mandrel will result in the most non-uniform and thinnest necks.

Necking down from .308 using a die(s) will give thickest neck brass. Necks will form crooked using a single pass through a 260/7-08 die but will be adequate for any factory app. Necking down using a stepped neck bushing will yield straight necks but is quite expensive. Necking down from 308 will generally result in a donut.

All are "doable" but I'd probably choose to neck down the .308 brass if it's a factory rifle. I suggest you get your new die and just cram the cases thru it, one pass and you're shooting. 99% of the world will be made happy by this procedure.

I've tried it both ways and prefer using .243 brass. (assuming there is no possiblity of a .260 round being loaded in a .243) If you've a factory rifle probably either way would work but my first go at this sort of thing was with a custom barreled rifle in 7-08and .308 brass had to be neck turned while .243 didn't.
I was able to load full power loads from the get go with the resized brass and I guess I'm in the 99% because I found the accuracy to be more than very good.

I am in process of buying a Browning A-Bolt titanium offered in 7mm-08. Plans are to shoot "as is" for the time being, but possibly custom rebarrel to .260 Remington or .260 Remington AI in the future. Weird huh ?

Years ago I bought a set of Redding forming dies for reforming 30 caliber mag cases to 7mm Remington mag and they worked pretty slick.

I have not checked yet, but there most likely is a forming die set for application of .308 Winchester to either a .260 Remington - or - 7mm-08 Remington offered by Redding.

Although others have suggested just using a full length die, my past experience forming magnum cases has not produced satisfactory results, but most likely was something I was - or - was not doing during forming process.

I would cull out a handful of 308 cases with high thickness variance, and neck turn trial & error for settings that produce a good thickness after downsizing. If you turn up onto the neck shoulder a bit & with downsizing, there won't be any doughnut. You can also leave your false shoulder for fireforming headspace. I would improve a 260, as there is plenty that can be in stock form.
I suppose now you have your pick of small or large primer in 308 brass. But I'd stay with large for this capacity..

I load 243 and 260 for my granddaughter. The 260 came first and I prefered to neck up 243 brass to 260. 308 was a little shorter than 260 min trim length and it made the neck pretty thick. She now has a 243 also, so I no longer resize either. I keep all brass sized to the headstamp to minimize the chance of her shooting a 243 bullet in her 260 or try and load a 260 in her 243.